This has been on my to-buy list for a while. Something I should probably do, because while recovery from the built-in recovery interface is fine, having an offline bootable backup is also great. It also doesn't interfere with having Time Machine be the "standard" backup.

I could probably setup a calendar appointment to dump a bootable image once a month to an external disk.

You can just use the UI to make whatever schedule you want (monthly, daily, every Monday, etc.). I think it edits your crontab behind the scenes. I set it to daily, but you could set whatever you want. You can even have multiple schedule entries, similar to cron.

Edit: Yeah, the bootable backups have saved me more than once. It's great to just be able to keep working even when the system disk is kaput.